The image quality provided by a C-MOS image sensor depends on multiple parameters two of which are particularly important: the level of noise and the dynamic operating range. A low level of noise produces an exploitable image in a low level of light, while an extended dynamic range improves quality in diurnal conditions, especially sunny. The dynamic range also conditions the tolerance of the sensor relative to the light spread in the same scene and the sudden variation in luminosity between the successive images.
Today, the level of noise of a CMOS charge transfer pixel, often called four-transistor pixel or simply 4T pixel, such as illustrated schematically in FIG. 1A, is almost reduced to the physical limit. This proper performance is essentially due to the use of a pinned photodiode (PPD), initially completely depleted, linked to a charge transfer gate to a floating diffusion node FD. Two correlated readings, performed before and after transfer of charges to the floating diffusion node, enables near-total cancelling of the reset noise (KTC noise proportional to capacitance).
Given that the pinned photodiode contains no mobile charges at the start of photo-conversion and after its transfer, it does not contribute to generation of KTC noise. Also, the photoelectric charge is measured on the floating diffusion FD, whereof the capacitance is dissociated from that of the photodiode. Accordingly the pinned photodiode collects the photoelectric charge only, which is then measured on the floating diffusion after transfer.
Low capacitance of the floating diffusion substantially increases variation in voltage by each transferred photoelectron, and this improves the overall signal-to-noise ratio, as the noise of the system remains relatively constant. For example, for background noise of a system at 160 μV, for a capacitance of 5 fF giving a variation in voltage of 32 μV per electron, this noise system equals 5 electrons on the floating diffusion FD. And with a capacitance of 1 fF for the floating diffusion FD, a background noise of 160 μV is reflected by 1 electron only.
The pixel structure with charge transfer has good sensitivity via a low value in capacitance of the floating diffusion FD and an effective charge transfer device, but the operating dynamic is reduced as a low capacitance of the floating diffusion zone FD prevents it from receiving large numbers of charges. For example, a capacitance of the floating diffusion zone FD of 5 fF on 1V of variation in voltage gives 31250 electrons, whereas 1 fF on 1V gives 6250 electrons only. This means that a 4T cell saturates very quickly. All current efforts are put into boosting the integration capacitance of the PPD and the storage capacitance of the floating diffusion FD.
The variation in voltage generated by an electron on a floating diffusion FD is defined as a conversion factor or gain. For example, a capacitance of the floating diffusion FD of 5 fF gives a conversion factor of 32 μV/e. The number of charges accepted by a pixel in its linear operating range is known as “Full Well Capacitance” (FWC). The FWC of a 4T pixel is limited either by the storage capacitance of the pinned photodiode or by the maximum number of charges acceptable by the floating diffusion FD.
The pinned photodiode PPD of FIG. 1A comprises as is known per se by N-doping in a substrate P, N-doping being covered by a fine layer of P-doping at very high dose. When the zone N (cathode) of the pinned photodiode is reverse-biased at a sufficiently high voltage, this zone N is completely depleted of mobile charges (electrons). This voltage noted as Vpin is called “pinning voltage”; it conditions the storage capacitance of photoelectrons per unitary surface in the photodiode. This explains use of strong surface doping which pushes the depletion zone as far as possible into the zone N to increase the value of the FWC capacitance for the same value Vpin. The voltage Vpin in a conventional pixel concept is fixed in general around 1V.
If the conversion factor of such a 4T pixel is relatively easy to increase due to the fineness of the etching used, the value of the FWC capacitance is more difficult to retain in the course to the resolution where the size of each pixel becomes smaller and smaller. It is simply clear that the ultimate physical limit of the FWC capacitance is the number of doping atoms in the photodiode. The low supply voltage in a C-MOS pixel results in a relatively low doping level and fairly restricted doping volume. It is therefore difficult to attain an operating dynamic of a 4T pixel beyond 60-70 dB (factor 1000). Applications such as video surveillance, automotive vision, etc. need a dynamic range of the order of 120 dB (factor 1,000,000).
Different approaches have been envisaged for increasing this dynamic, as summarized hereinbelow.
Document U.S. Pat. No. 6,921,934B2 proposes a structure for double pinned photodiode for increasing the storage capacitance of the photodiode.
Document U.S. Pat. No. 6,677,656B2 proposes a pinned photodiode with a P-layer for increasing the integration capacitance of the photodiode.
Document U.S. Pat. No. 7,705,900B2 proposes selectively combining severally floating diffusions for increasing the capacitance of the floating diffusion at a high lighting level.
Document WO2004/112376A1 proposes modulating the capacitance of the floating diffusion to adjust the conversion factor and the charge-measuring capacitance.
Document WO2012/092194A1 proposes modulation of the capacitance of the floating diffusion by programming voltage.
Document WO2007/021626A2 proposes selectively adding additional capacitance to the floating diffusion for increasing the charge-measuring capacitance.
It is reminded that document EP1354360A1, having the same inventor as that of the present patent application, describes a pixel with a photodiode in solar cell mode, producing a logarithmic response as a function of the light intensity. With the same inventor, patents EP2186318A1 and WO2010/103464A1 contribute improvements of compactness and power consumption. Sensors made according to these documents give an excellent operating dynamic in exhibiting almost no saturation even in very strong illumination. The operating dynamic largely exceeds 120 dB.
But these known pixel structures present a sensitivity which can be perfected and with difficulty can cover needs in low-level light imposed by applications such as surveillance. There are two reasons for this low sensitivity:
1) Substantial junction capacitance of the photodiode, giving a low conversion factor; but if the photodiode size is increased, it accepts more charges of photoelectric origin but the junction capacitance of the photodiode is almost increased in the same proportion. The amplitude of the signal remains unimproved. This low conversion factor amplifies the impact of noises present in the system.
2) The presence of KTC noise during reset: in such a logarithmic pixel the noise induced by the reset operation is linked to the value of the junction capacitance of the photodiode and cannot be compensated; for example, a junction capacitance of 10 fF gives a KTC noise of 40 electrons, so the noise situation in such a pixel is very close to that of a classic active pixel having 3 transistors. Compared to a 4T pixel where the KTC noise is totally suppressed, the distance is enormous in low-level light.